Moves the Array forwarding constructors and _ArraySize class out of
the patch files so that the analyzer can reason about them.
(The analyzer does not include patch files, because it does not know
whether it's analyzing for VM or JS backend.)
Closes: https://github.com/dart-lang/sdk/issues/45507
Change-Id: Ia0eb4bbca40b906a1e636714f53c5c73dcfc3cad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193403
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>